1. Field of the Invention
The invention relates to a fuel delivery system for an internal combustion engine for delivering fuel at high pressure to the injector valves of the engine.
2. Brief Description of the Prior Art
From German Patent Disclosure DE 195 39 885 A1, a fuel delivery system for an internal combustion engine is already known which has a fuel feed pump and connected in series with it a high-pressure fuel pump, so that fuel at high pressure can be furnished from the high-pressure side of the high-pressure fuel pump, via a pressure line, a reservoir and valve lines, to injection valves, each of which injects fuel directly into one of the combustion chambers of the engine. The fuel feed pump, whose outlet side communicates with the low-pressure side of the high-pressure fuel pump via a pressure line, furnishes fuel that is at pilot pressure to the high-pressure fuel pump.
To keep the pilot pressure in the pressure line at a desire value, a pressure limiting valve is connected to the pressure line via a 2/2-way valve, which either blocks or opens the communication between the pressure line and the pressure limiting valve.
To compensate for the low pumping capacity of the high-pressure fuel pump during the engine starting phase and optionally to scavenge the pressure line on the high-pressure side and the adjoining reservoir so as to enable removing gas bubbles that are created while the engine is stopped, an admission device is provided parallel to the high-pressure fuel pump and connects the low-pressure side and the high-pressure side of the high-pressure fuel pump to one another. To raise the pilot pressure in the pressure line on the low-pressure side to 8-10 bar during the starting phase, compared with the pilot pressure during normal operation, the 2/2-way valve can be closed, so that no fuel can flow out of the pressure line. The elevated pilot pressure during the starting phase makes it possible on the one hand to scavenge the fuel delivery lines to eliminate gas bubbles and on the other to compress gas bubbles, as well as enabling a high pumping capacity that is suitable for a starting event.
During normal operation of the engine, the injection pressure is generated in the reservoir by the high-pressure fuel pump and is limited by a controllable pressure regulating valve to an appropriate value. To that end, the pressure regulating valve communicates with the low-pressure side via a return line.
However, a limitation of the temperature of the high-pressure fuel pump is effected at best only by a certain cooling by means of the fuel flow through the high-pressure fuel pump, so that it cannot reliably be prevented that the high-pressure fuel pump will heat up enough that its temperature exceeds the critical operating temperature, that is, the temperature at which, for a given pilot pressure, fuel vapor bubble development begins.
In another fuel delivery system, in which a high-pressure fuel pump for supplying direct injection valves is supplied with fuel at pilot pressure by a fuel feed pump, it is provided that the pressure line connecting the pumping side of the fuel feed pump to the low-pressure side of the high-pressure fuel pump communicates via a variable throttle valve with a first pressure limiting valve for a first, relatively low pressure, such as 3 bar, and communicates directly with a second pressure limiting valve for a relatively high pilot pressure, such as 9 bar. The variable throttle valve has a flow resistance which increases disproportionately as the flow rate increases, so that the pilot pressure in the pressure line can be adjusted by means of the pumping capacity of the fuel feed pump.
In order to prevent vapor bubble development in the high-pressure fuel pump when the fuel temperature is rising, it is possible in this fuel delivery system, by increasing the pumping capacity of the fuel feed pump, to raise the pilot pressure such that it becomes greater than the temperature-dependent vapor pressure of the fuel in the pressure line.
In this way, it is true that the vapor bubble development in the fuel and hence a drop in the pumping capacity of the high-pressure fuel pump, which would make any further buildup of high pressure impossible can indeed be prevented. However, the fuel feed pump would be stressed considerably by such an operating mode, which would reduce its service life.
From German Patent Disclosure DE 38 36 507 A1, for cooling a control motor of a throttle valve adjusting unit it is known for a flow of coolant water for the control motor to be diverted from the engine coolant system.